incorrect gradient in linear color ramp
The value is linear:



Human vision is not linear. Preceived value of a diffuse surface reflecting 50% of light looks brighter than middle grey to us. Humans preceive around 21% or 18% reflected light as middle grey depending on who you ask. The Value in HSV is based on preception. 

If you used Gamma node with 2.2 after the gradient it would output approximately linear greadient based on human preception.